Again I don't know what type of metal this is. I never had it checked out by a jeweler. Your guess is as good as mine. There are no markings in side so I'm betting costume. Hope that helps.
So I have no idea what type of metal. As far as size for a 12 year girl I don't know. I do know that it fits my ring finger. It really depends on her finger size. Most likely will fit her middle finger if I were to guess.